Si based light emission

Si based light emission from Rare earth (RE) implanted MOS devices

Rare earth (RE) elements are already successfully used in a couple of optic and optoelectronic applications like lasers, phosphors and plasma displays. They feature narrow emission lines in the ultra-violet, the visible and infrared spectral region which originate from 4f inner shell transitions.

Embedding RE elements in SiO2 has a couple of significant advantages:

SiO2 is a mechanically and chemically robust dielectric.

The processing steps required to fabricate an efficient light emitter on the basis of RE elements implanted in SiO2 are fully compatible with current Si technology.

Size and shape of the light emitters is only limited by the available photolithography and can be tailored to the need of the specific application (see right)

Strong Electroluminescence from RE-implanted MOS devices with various shapes

Basic preparation scheme

Oxidation: In order to fabricate a stable device, the quality of the oxide is of major importance. In our group we are using the LOCOS (Local oxidation of silicon) technology to avoid early breakdowns of the devices.

Ion dose and energy mainly determine the spectrum and the efficiency of the light emitter.

The annealing conditions both influence the efficiency and the stability of the devices. The annealing must provide a sufficient high temperature to remove or to diminish the defects produced by implantation, and the annealing time must be short enough to avoid diffusion and clustering of the implanted RE elements. For this reason Rapid thermal Annealing (RTA) or Flash lamp annealing (FLA)[1] is used.

Electroluminescence properties

Electroluminescence spectra from various RE-implanted MOS devices. The transition of the main emission line is given in the diagrams.

The 4fn configuration (with n electrons in the 4f shell) is well screened from the chemical environment resulting in sharp emission lines originating from 4f inner shell transitions. One exception is Ce showing a broad emission line due to an electronic transition from the 5d to the 4f shell.

As transitions within the 4f shell are electric dipole-forbidden, the luminescence obtained by direct photoexcitation is relatively weak. Fortunately, the RE luminescence centers can be efficiently excited by non-photonic processes like energy transfer or direct excitation by hot electrons. The latter process occurs in the case of RE implanted MOS devices, and strong electroluminescence from the various devices can be observed.

The EL is well visible with the naked eye at daylight, and in the case of Tb-implanted MOS devices we were able to achieve an external quantum efficiency of 16 % which is equivalent to a power efficiency of 0.3 %. A maximum luminance above 2800 cd/m2 and a luminous efficiency exceeding 2.1 lm/W were observed.

In the most cases the EL spectrum doesn’t change very much with increasing injection current. An interesting exception is Eu, where the dominance of the blue or the red peak depends on the excitation condition. This property can be exploited to construct a device which can switch between two states of luminescence: either red or blue EL.

Excitation Mechanism

The excitation mechanism of the RE luminescence centers comprises the injection of electrons, their transport through the oxide layer and the excitation of the RE ions.

Direct tunnelling of electrons

Trap assisted tunnelling

Acceleration of electrons. For electric fields in the order of 10 MVcm-1 the electrons have an average kinetic energy of 4-5 eV

Excitation of RE ions by inelastic scattering events

Hopping or Pool-Frenkel conduction (does not contribute to luminescence)

Electron trapping leading both to a blocking of luminescence centers and a buildup of space charges which counteracts the further injection of electrons.

Band-to-band impact excitation leading to hole generation and a continuous degradation of the oxide

Applications

The MOS structure with RE offers an excellent integrability into the common Si technology, the possibility to tailor the size and the design of the light sources to the specific need of the application and considerable cost savings in mass production. Based on these advantages this type of light source is suitable for the fluorescence analysis in sensor systems, especially for microarrays (see figure). The use of such light emitter arrays allows a considerable shrinking of the measurement apparatus dimensions which is of special interest for point-of-care applications.

Problems of conventional systems:

The light of a laser is guided through a complex optical system to the biochip.

The fluorescence light emitted from the dye is normally collected by a CCD camera.

In order to scan all dots either the optical system or the sample has to be moved mechanically with high precision.

The light collected by a CCD camera splits into a large number of pixels. To record low signals, data binning at the expense of lateral resolution has to be performed

Advantages of miniaturized light sources:

No laser / optical system is needed.

No mechanical movement is necessary.

The lateral resolution is automatically given by a sequential operation of the light sources.

No crosstalk between different dots can occur.

The light can be detected by a simple, inexpensive but high-quality and large area Silicon detector.

The detector records an integral signal and can trace even very low light intensities.